1. Field of the Invention
The present invention relates to a speaker apparatus for acoustic reproduction and a sound reproduction system employing the same.
2. Description of the Related Art
Various types of speakers for acoustic reproduction have been conceived and made practical.
Speaker units have been practically formed as electromagnetically coupled (electromagnetically induced type) speakers in which, for example, a magnet is sandwiched between a center pole portion provided in a yoke and a plate, forming a magnetic circuit having a gap between the center pole portion and the plate, within the gap of the magnetic circuit, a primary coil is fixed to the center pole portion or the plate, and a secondary coil which forms a short coil is disposed within the gap of the magnetic circuit in such a manner as to be fixed to a vibration plate so as to face the primary coil.
In this electromagnetically coupled speaker, a secondary electric current is induced in the secondary coil by a signal current flowing through the primary coil. Due to the interaction with the magnetic flux which occurs in the gap of the magnetic circuit, a driving force responsive to the secondary electric current is produced in the secondary coil in accordance with Fleming's left-hand rule, causing the vibration plate to which the secondary coil is fixed to deflect. In this way, the vibration plate is moved, thereby generating a sound.
This electromagnetically coupled speaker has the advantages of having excellent heat dissipation properties and the capability of withstanding a large input because the primary coil through which a signal current flows is fixed to a center pole portion or a plate formed from a magnetic material, such as iron. Further, if the secondary coil which forms a short coil is formed from a non-magnetic conductive material, for example, a cylindrical member for the length of one turn formed from, for example, aluminum, distortion can be reduced.
On the other hand, a dynamic (electroconductive type) speaker having a voice coil disposed within a gap in a magnetic circuit is made practical. In this dynamic speaker, electric power is supplied to a voice coil, and the voice coil is connected to an input terminal provided in a speaker frame by means of a coil extension wire made of tinsel wire so that unwanted vibration and resistance are not applied to the vibration system including the voice coil.
Further, in this dynamic speaker, it is considered that the voice coil is divided into portions corresponding to the number of bits of a digital sound signal, and that the respective coils are directly driven by data of the corresponding respective bits of the digital sound signal.
As described above, the electromagnetically coupled speaker has the advantages of having excellent heat dissipation properties and the capability of withstanding a large input, and further is capable of reducing distortion. However, if the width of the gap in the magnetic circuit is increased, the magnetic sensitivity of the primary coil and the secondary coil is decreased; therefore, it is not possible to increase the number of turns of the primary coil and the secondary coil.
For this reason, it is not possible to increase the inductances of the primary coil and the secondary coil, and the electromagnetic coupling force by which a secondary electric current is induced in the secondary coil by the signal current flowing through the primary coil is reduced at a low frequency of below several kHz. Therefore, reproduction of, for example, from 1 kHz to 20 Hz required for sound reproduction cannot be sufficiently made. Due to this, the electromagnetically coupled speaker is used mainly as a speaker for reproducing high-pitched sounds.
On the other hand, as described above, in a dynamic speaker, a voice coil is connected to an input terminal provided in the speaker frame by means of a coil extension wire made of tinsel wire. Further, in the dynamic speaker, it is considered that the voice coil is divided into portions for the number of bits of a digital sound signal, and that the respective coils are directly driven by data from each bit of the digital sound signal.
However, at present, in a case where a sound signal is digitized, it is common practice to form the digital sound signal with 16 bits for the purpose of faithful sound reproduction. For this reason, in a dynamic speaker, when a voice coil is driven in accordance with a digital sound signal, 16 pairs (i.e., 32 wires) of coil extension wires become necessary for one speaker.
However, since the tinsel wire, which is a coil extension wire, greatly swings with the vibration of the voice coil because the tinsel wire is extended from a moving object, namely, a moving voice coil, it is not possible to decrease the distance between them. Therefore, it is very difficult to provide as many as 32 tinsel wires in a speaker. In particular, it is difficult to manufacture a small-size speaker.